Longitudinal field driven ion mobility filter and detection system

ABSTRACT

An asymmetric field ion mobility spectrometer for filtering ions via an asyretric electric field, an ion flow generator propulsing ions to the filter via a propulsion field.

RELATED APPLICATION(S)

[0001] This application is a Continuation-In-Part of U.S. applicationSer. No. 09/439,543 filed Nov. 12, 1999, which is a Continuation-In-Partof U.S. application Ser. No. 09/358,312 filed Jul. 21, 1999, all ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to chemical analytical systemsbased on ion mobility, and conveyance of ions through such a system.

BACKGROUND OF THE INVENTION

[0003] The ability to detect and identify explosives, drugs, chemicaland biological agents as well as monitor air quality has becomeincreasingly more critical given increasing terrorist and militaryactivities and environmental concerns. Previous detection of such agentswas accomplished with conventional mass spectrometers, time of flightion mobility spectrometers and conventional field asymmetric ionmobility spectrometers (FAIMS).

[0004] Mass spectrometers are very sensitive and selective with fastresponse time. Mass spectrometers, however, are large and requiresignificant amounts of power to operate. They also require a powerfulvacuum pump to maintain a high vacuum in order to reduce ion neutralinteractions and permit detection of the selected ions. Massspectrometers are also very expensive.

[0005] Another spectrometric technique which is less complex is time offlight ion mobility spectrometry which is the method currentlyimplemented in most portable chemical weapons and explosives detectors.The detection is based not solely on mass, but on charge andcross-section of the molecule as well. However, because of thesedifferent characteristics, molecular species identification is not asconclusive and accurate as the mass spectrometer. Time of flight ionmobility spectrometers typically have unacceptable resolution andsensitivity limitations when attempting to reduce their size. In time offlight ion mobility, the resolution is proportional to the length of thedrift tube. The longer the tube the better the resolution, provided thedrift tube is also wide enough to prevent all ions from being lost tothe side walls due to diffusion. Thus, fundamentally, miniaturization oftime of flight ion mobility systems leads to a degradation in systemperformance. While conventional time of flight devices are relativelyinexpensive and reliable, they suffer from several limitations. First,the sample volume through the detector is small, so to increasespectrometer sensitivity either the detector electronics must haveextremely high sensitivity, requiring expensive electronics, or aconcentrator is required, adding to system complexity. In addition, agate and gating electronics are usually needed to control the injectionof ions into the drift tube.

[0006] FAIMS spectrometry was developed in the former Soviet Union inthe 1980's. FAIMS spectrometry allows a selected ion to pass through afilter while blocking the passage of undesirable ions. But the onlycommercial prior art FAIMS spectrometer was large and expensive, e.g.,the entire device was nearly a cubic foot in size and cost over $25,000.Such systems are not suitable for use in applications requiring smalldetectors. They are also relatively slow, taking as much as one minuteto produce a complete spectrum of the sample gas, are difficult tomanufacture and are not mass producible.

[0007] The prior art FAIMS devices depend upon a carrier gas that flowsin the same direction as the ion travel through the filter. However, thepumps required to draw the sample medium into the spectrometer and toprovide a carrier gas can be rather large and can consume large amountsof power.

[0008] It is therefore an object of the present invention to provide anion filter and detection system which does not require the high flowrate, high power consumption pumps normally associated with FAIMSspectrometers.

[0009] It is another object of the present invention to provide methodand apparatus for highly efficient conveyance of ions into and through ahigh field ion mobility filter.

[0010] It is a further object of the present invention to provide methodand apparatus for efficient conveyance of ions into and through a highfield ion mobility filter without the use of a carrier gas.

[0011] It is another object of the present invention to provide a FAIMSfilter and detection system which can quickly and accurately control theflow of selected ions to produce a sample spectrum.

[0012] It is a further object of the present invention to provide aFAIMS filter and detection system which has a sensitivity of parts perbillion to parts per trillion.

[0013] It is a further object of the present invention to provide aFAIMS filter and detection system which may be packaged in a singlechip.

[0014] It is a further object of the present invention to provide aFAIMS filter and detection system which is cost effective to implement,produce and operate.

SUMMARY OF THE INVENTION

[0015] The present invention features an ion mobility spectrometer forfiltering ions via an asymmetric electric field. Ions are transportedalong the longitudinal ion flow path via an ion flow generator. The ionflow generator preferably provides ion propulsion via a local electricfield in the flow path. Operation of the invention enables eliminationor reduction of flow rate and power requirements of conventional gasflow.

[0016] In a preferred embodiment, a longitudinal electric fieldgenerated by the ion flow generator propels ionized sample received froman ionization region through a compensated, asymmetric electric field ofthe ion filter, with a desired species passing through the filter andflowing toward a detector region. Various options are possible. In oneembodiment, a low volume gas flow carries the sample to the filter. Inother embodiment, there is no need for gas flow and ion steering, or thelongitudinal field itself, propels ions into the filter region, wherethe ions are further propelled by the ion flow generator.

[0017] In another embodiment, a supply of clean filtered air is flowedin the negative longitudinal direction opposite the desired direction ofion flow to keep the ion filter and detector regions free of neutralsand to help remove solvent, reduce clustering, and minimize the effectsof humidity.

[0018] A preferred embodiment of the present invention features an ionmobility spectrometer having a housing structure that defines a flowpath (also known as a drift tube) that begins at a sample inlet forreceipt of sample (i.e., sample molecules to be analyzed) and brings thesample to an ionization region. Once ionized, the sample passes to theion filter, with desired ion species passing through the filter in theflow path, as propelled by the ion flow generator.

[0019] In one embodiment, the ion filter is provided with a plurality ofhigh frequency, high voltage filter electrodes for creation of theasymmetric electric field transverse to the longitudinal ion flowdirection along the flow path. In a preferred embodiment, this field iscompensated, to pass only a desired ion species for downstreamdetection. In another embodiment, filtering is trajectory based withoutrequiring compensation.

[0020] The ion flow generator creates a longitudinal electric fieldalong the flow path (transverse to the asymmetric electric field) forpropelling or transporting the ions through the asymmetric electricfield toward the output region to enable detection and analysis. Theionization source may include a radiation source, an ultraviolet lamp, acorona discharge device, electrospray nozzle, plasma source, or thelike.

[0021] In one embodiment, an electric controller supplies a compensationbias and an asymmetric periodic voltage to the ion filter. The ionfilter typically includes a pair of spaced electrodes for creating theasymmetric electric field between the electrodes. The ion flow generatortypically includes a plurality of spaced discrete electrodes proximateto the filter electrodes for creating a longitudinal direction electricfield which propels the ions through the transverse asymmetric electricfield, and onward for detection. The ion filter and flow generator mayshare none, some or all electrodes.

[0022] In another embodiment, the ion flow generator includes spacedresistive layers and a voltage is applied along each layer to create thelongitudinally directed electric field which propels the ions throughthe filter's compensated asymmetric electric field and to the detector.

[0023] In another embodiment, the ion filter includes a first pluralityof discrete electrodes electrically connected to an electric controllerwhich applies the asymmetric periodic voltage to them. The ion flowgenerator includes a second plurality of discrete electrodes dispersedamong the electrodes of the ion filter and connected to a voltage sourcewhich applies a potential gradient along the second plurality ofdiscrete electrodes. Compensation voltage applied to the filter opensthe filter to pass a desired ion species if present in the sample. Ifthe compensation voltage is scanned, then a complete spectrum of thecompounds in a sample can be gathered.

[0024] In one embodiment, the ion filter includes electrodes on aninside surface of the housing and the ion flow generator includeselectrodes proximate to the ion filter electrodes. The housing may beformed using planar substrates. The ion detector also includeselectrodes on an inside surface of the housing proximate to the ionfilter and the ion flow generator.

[0025] In another embodiment, the ion filter may include electrodes onan outside surface of the housing and the ion flow generator thenincludes resistive layers on an inside surface of the housing. A voltageis applied along each resistive layer to create a longitudinal electricfield. Alternatively, the ion filter and the ion flow generator arecombined and include a series of discrete conductive elements eachexcited by a voltage source at a different phase.

[0026] In another embodiment, both the longitudinal and transversefields and voltages are applied or generated via the same electrodes orvia members of a set of electrodes. Because of the flexibility of theelectronic drive system of the invention, all or part of the electrodeset may be used for a given function or more than one function in seriesor simultaneously.

[0027] In yet a further embodiment of the invention, filtering isachieved without compensation of the filter field. In one practice, thespectrometer has a single RF (high frequency, high voltage) filterelectrode on a first substrate, and a plurality of multi-functionelectrodes on a second substrate that are formed facing the filterelectrode over the flow path. The plurality of electrodes forms asegmented detector electrode. Ions are filtered and detected bytrajectory, being controlled by the asymmetric field and landing on anappropriate one of the detector electrode segments. Thus filtering isachieved without compensation of the filter field in a very compactpackage. The detector electrodes are monitored, wherein a particularspecies can be identified based on its trajectory for a given detectionand given knowledge of the signals applied, the fields generated, andthe transport (whether gas or electric field).

[0028] In practice of the invention, prior art pumps used to draw asample, such as a gas containing compounds to be analyzed, into a FAIMSspectrometer, and to provide a flow of carrier gas, can be made smalleror even eliminated in practice of the invention. This is enabled inpractice of the invention by incorporation of an ion flow generatorwhich creates a longitudinal electric field in the direction of theintended ion travel path to propel the ions toward a detector regionafter passing through a transversely directed asymmetric electric fieldwhich acts as an ion filter.

[0029] The result is the ability to incorporate lower cost, lower flowrate, and smaller, even micromachined pumps, in embodiments of theinvention; a decrease in power usage; the ability to apply cleanfiltered gas (e.g., dehumidified air) in a direction opposite thedirection of ion travel to eliminate ion clustering and the sensitivityof the spectrometer to humidity. Separate flow paths for the source gasand the clean filtered gas may not be required, thus reducing thestructure used to maintain separate flow paths taught by the prior art.Moreover, if an electrospray nozzle is used as the ionization source,the electrodes used to create the fine droplets of solvent can beeliminated because the electrodes which create the longitudinal andtransverse electric fields can be used to function both to transport theions and to create the fine spray of solvent droplets.

[0030] In a practice of the invention, an extremely small, accurate andfast FAIMS filter and detection system can be achieved by defining anenclosed flow path between a sample inlet and an outlet using a pair ofspaced substrates and disposing an ion filter within the flow path, thefilter including a pair of spaced electrodes, one electrode associatedwith each substrate and a controller for selectively applying a biasvoltage and an asymmetric periodic voltage across the electrodes tocontrol the path of ions through the filter. In a further embodiment ofthe invention, it is possible to provide an array of filters to detectmultiple selected ion species.

[0031] Alternative filter field compensation in practice of embodimentsof the invention may be achieved by varying the duty cycle of theperiodic voltage, with or without a bias voltage. Furthermore, in anembodiment of the invention, it is possible that by segmenting thedetector, ion detection may be achieved with greater accuracy andresolution by detecting ions spatially according to the ions'trajectories as the ions exit the filter.

[0032] It will be further understood that while ion travel within theion filter is determined by the compensated asymmetric filter field andthe ion transport field, the invention may also include an ionconcentrating feature for urging ions toward the center of the flowpath. In one embodiment this concentrating is achieved where fieldsbetween electrodes on each substrate work together to urge the ionstoward the center of the flow path as they pass there betweenapproaching the ion filter.

[0033] In other embodiments, ion filtering is achieved without the needfor compensation of the filter field. In one illustrative embodiment, aspectrometer of the invention has preferably a single RF (highfrequency, high voltage) filter electrode. A segmented filter-detectorelectrode set faces the first electrode over the flow path, with thefilter-detector electrode set having a plurality of electrodes in a rowmaintained at virtual ground. The asymmetric field signal is applied tothe filter electrode and the asymmetric field is generated between thefilter electrode and the filter-detector electrode set. Ions flow in thealternating asymmetric electric field and travel in oscillating pathsthat are vectored toward collision with a filter electrode, and inabsence of compensation, favorably enables driving of the ions tovarious electrodes of the filter-detector electrode set. Thesecollisions are monitored.

[0034] In a further embodiment, upstream biasing effects which ions flowto the filter. For example, a sample flows into an ionization regionsubject to ionization source, and electrodes are biased to deflect andaffect flow of the resulting ions. Positive bias on a deflectionelectrode repels positive ions toward the filter and attractingelectrodes being negatively biased attract the positive ions into thecentral flow of the ion filter, while negative ions are neutralized onthe deflection electrode and which are then swept out of the device.Negative bias on the deflection electrode repels negative ions towardthe filter and attracting electrodes positively biased attract thenegative ions into the central flow path of the filter, while positiveions are neutralized on the deflection electrode.

[0035] In an embodiment, the path taken by a particular ion in thefilter is mostly a function of ion size, cross-section and charge, whichwill determine which of the electrodes of the filter-detector electrodeset that a particular ion species will drive into. This speciesidentification also reflects the polarity of the ions and the high/lowfield mobility differences (“alpha”) of those ions. Thus a particularion species can be identified based on its trajectory (i.e., whichelectrode is hit) and knowledge of the signals applied, the fieldsgenerated, and the transport characteristics (such as whether gas orelectric field).

[0036] In practice of the filter function of the invention, where theupstream biasing admits positive ions into the filter, those positiveions with an alpha less than zero will have a mobility decrease with anincrease of a positively offset applied RF field. This will affect thetrajectory of these ions toward the downstream detector electrodes.However, a positive ion with an alpha greater than zero will have amobility increase with an increase of a negatively offset applied RFfield, which in turn will shorten the ion trajectory toward the nearerdetector electrodes.

[0037] Similarly, where the filter received negative ions, a negativeion with an alpha less than zero will have a mobility increase with anincrease of a positively offset applied RF field; this will tend toaffect the ion trajectory toward the downstream detector electrodes.However, a negative ion with an alpha greater than zero will have amobility increase with an increase of a negatively offset applied RFfield, which in turn will tend to shorten the ion trajectory toward thenearer detector electrodes. Thus, ions can be both filtered and detectedin a spectrometer of the invention without the need for compensation.

[0038] In various embodiments of the invention, a spectrometer isprovided where a plurality of electrodes are used to create a filterfield and a propulsion field, in a cooperative manner that may befeature simultaneous, iterative or interactive use of electrodes. Wherea plurality of electrodes face each other over a flow path, the filterfield and the propulsion field may be generated using the same ordifferent members of the electrode plurality. This may be achieved in asimple and compact package.

[0039] In practice of the invention, a spectrometer is provided invarious geometries where a plurality of electrodes are used to create afilter and a propulsion field, in a cooperative manner that may besimultaneous or interactive. Where a plurality of electrodes face eachother over a flow path, the filter field and the propulsion field may begenerated using the same or different members of the electrode pluralityto pass selected ion species through the filter.

[0040] It will be appreciated that in various of the above embodiments,a spectrometer can be provided in any arbitrarily shaped geometry(planar, coaxial, concentric, cylindrical) wherein one or more sets ofelectrodes are used to create a filtering electric field for iondiscrimination. The same or a second set of electrodes, which mayinclude an insulative or resistive layer, are used to create an electricfield at some angle to the filtering electric field for the purpose ofpropelling ions through the filtering field to augment or replace theneed for pump-driven propulsion such as with a carrier gas.

[0041] It will now be appreciated that a compact FAIMS spectrometer hasbeen provided with e-field ion propulsion. Benefits of the inventioninclude provision of a stable, easily controlled ion flow rate withoutthe need for gas flow regulation. Elimination of the need for gas flowregulation reduces complexity and cost and improves reliability.Dramatic reduction of gas flow substantially reduces power consumption.Operation of the invention can reduce the amount of sample neutralsentering the analysis region between the filter electrodes. If only ionsare injected into the filter, then it is easier to keep the ion filterin a controlled operating state, such as control of moisture level. Theresult is very reproducible spectra in a low power analytical system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

[0043]FIG. 1 is a schematic block diagram of a PFAIMS filter anddetection system according to the present invention.

[0044]FIG. 2 is a schematic representation of the ions as they passthrough the filter electrodes of FIG. 1 toward the detector.

[0045]FIGS. 3A, 3B provide graphical representation of an asymmetricperiodic voltage having a compensating varying duty cycle, for filteringunwanted ions (FIG. 3A) and passing through the filter selected ionspecies (FIG. 3B) without a bias voltage.

[0046]FIG. 4 is a schematic diagram of a segmented detector embodimentof the invention.

[0047]FIGS. 5A, 5B are graphical representations of the spectrometerresponse to varying concentrations of acetone and di-ethylmethyl aminein an embodiment of the invention.

[0048]FIG. 6 is a cross sectional view of a spaced, micromachined filterassembly according to an embodiment of the present invention.

[0049]FIG. 7 is a perspective view of a practice of the invention as apackaged micromachined filter and detection system, including pumps, inminiaturized format.

[0050]FIG. 8 is a cross sectional view of a dual channel embodiment ofthe invention.

[0051]FIG. 9 is a schematic view of a prior art spectrometer.

[0052]FIGS. 10-17 are respective schematic views of embodiments of thelongitudinal field driven ion mobility spectrometer of the presentinvention.

[0053]FIG. 18 is an embodiment of the invention that performs ionfiltering based on ion trajectory within the filter region.

[0054]FIG. 19 is a graphical representation of identification ofchemical constituents of a mixture (benzene and acetone) in practice ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0055] A description of preferred embodiments of the invention follows.

[0056] A preferred embodiment of the present invention provides methodand apparatus for conveyance of ions in and through an ion filterwithout the need for a carrier gas in an ion-mobility-based analyticalsystem. In embodiments of the present invention, the need for pumps iseither eliminated or the pumps are made smaller, even micromachined.Furthermore, separate flow paths for the source gas and the carrier gasare not required. In one filter embodiment, filtered gas is introducedto flow in a direction opposite the direction of ion travel to eliminateion clustering and to improve system sensitivity. Preferred andalternative embodiments of the invention are set forth below as anillustration and as a limitation.

[0057] A preferred planar FAIMS (PFAIMS) spectrometer 10, FIG. 1,operates by drawing a carrier gas 12 containing a sample S to beanalyzed (often collectively referred to as a gas sample), by means ofpump 14, through inlet 16 and into ionization region 17. The gas sampleis ionized by ionization source 18. Source 18 may include an ultravioletlight source, a radioactive device, plasma source, corona dischargedevice, electrospray head, or the like.

[0058] The ions 19 flow from the ionization region 17 along flow path 26into filter 24 defined by facing electrodes 20 and 22. As these ionspass between electrodes 20 and 22 they are exposed to an asymmetricelectric field 25 established between the filter electrodes, induced bya voltage applied from a source, such as voltage generator 28 directedby electronic controller 30. Filter field 25 is transverse to thelongitudinal flow of gas and ions along flow path 26.

[0059] The system is preferably driven by electronic controller 30,which may include, for example, amplifier 34 and microprocessor 36.Amplifier 34 amplifies the output of detector 32, which is a function ofthe charge collected by electrode 35 and provides the output tomicroprocessor 36 for analysis. Similarly, amplifier 34, shown inphantom, may be provided where electrode 33 is also utilized as adetector.

[0060] As part of the FAIMS filtering function, some compensation mustbe applied to the filter; which in turn selects a particular ion speciesthat will pass through the filter. In operation, as ions pass throughfilter field 25, some ions are neutralized as they travel into andcollide with filter electrodes 20 and 22. However the filter field iscompensated to bring a particular species of ion back toward the centerof the flow path, preventing it from being neutralized. Thus a desiredion species 19′ passes through the filter.

[0061] More specifically, as shown in FIG. 2, ions 19 flow in thealternating asymmetric electric field 25, in oscillating paths 42 a, 42b and 42 c. The time varying RF asymmetric voltage V applied to thefilter electrodes is typically in the range of ±(1000-10,000) volts andcreates electric field 25 with a maximum field strength of around 40,000V/cm. The path taken by a particular ion is mostly a function of itssize, cross-section and charge. Where the asymmetric field is notcompensated for the resulting high-low-field offset imposed on the ions,then the ions will reach and contact electrode 20 or 22 and will beneutralized. Thus as compensation is applied to the filter field, aparticular ion species will be returned back toward the center of theflow path and will pass through the filter for detection.

[0062] In a particular embodiment, compensation is achieved by applyinga compensation field 44, typically in the range of ±2000 V/cm from anapplied ±100 volt dc voltage, for example, applied concurrently andinduced at, adjacent to, or between, electrodes 20 and 22, via a biasvoltage applied thereto. Now a selected ion species 19′ passes throughfilter 24 for detection.

[0063] In one embodiment, compensation field 44 is a constant bias whichoffsets alternating asymmetric field 25 to allow the selected ionspecies 19′ to pass to detector 32. Thus, with the proper bias voltage,a particular species of ion will follow path 42 c while undesirable ionswill follow paths 42 a and 42 b to be neutralized as they encounterelectrode plates 20 and 22.

[0064] In an alternative practice of the invention, the duty cycle ofthe asymmetric periodic voltage applied to electrodes 20 and 22 offilter 24 is varied so that there is no need to apply a compensationvoltage. The control electronics varies the duty cycle of asymmetricalternating electric field 25, with the result that path of a selectedion species (defined mostly by charge and cross-section, among othercharacteristics, of the ions) is returned toward the center of the flowpath, and so to pass on for detection. As an example, and not by way oflimitation, the duty cycle of field 25 may be one quarter: 25% high,peak 70, and 75% low, valley 72; in which case, ions 19 on path 42 aapproach and collide with a filter electrode 20 and are neutralized(FIG. 3A). However, by varying the duty cycle to 40%, peak 70 a, 60%low, valley 72 a, ions 19′ on path 42 c pass through filter 24 andtoward the detector without being neutralized. Typically the duty cycleis variable from 10-50% high and 90-50% low (FIG. 3B). Accordingly, byvarying the duty cycle of field 25 an ion's path in field 25 may becorrected so that it will pass through filter 24 without beingneutralized and without the need for a compensating bias voltage.

[0065] Ions 19′ that pass through filter 24 are now delivered fordetection, which may be on-board or not. In a preferred embodiment, thedetector is on board-and is in the flow path. In one embodiment,detector 32 includes a biased top electrode 33 at a voltage and a biasedbottom electrode 35, possibly at ground, formed on the same substratesas the filter electrodes. Top electrode 33 can be set at the samepolarity as the ions to be detected and therefore deflects ions towardelectrode 35. However, either electrode may detect ions depending on thepassed ion species and bias applied to the electrodes. Moreover,multiple ions may be detected by using top electrode 33 as one detectorand bottom electrode 35 as a second detector.

[0066] The output of FAIMS spectrometer 10 is a measure of the amount ofcharge detected at detector 32 for a given RF field 25 and compensation.The longer the filter 24 is set at a given compensation level, the moreof a given species will be passed and the more charge will accumulate ondetector 32.

[0067] Alternatively, by sweeping compensation over a predeterminedvoltage range, a complete spectrum for the sample and gas can beachieved. A FAIMS spectrometer according to the present inventionrequires typically less than thirty seconds and as little as one secondor less to produce a complete spectrum for a given gas sample. Thus, byvarying compensation during a scan, a complete spectrum of the gassample can be generated.

[0068] To improve FAIMS spectrometry resolution even further, detector32, may be segmented, as shown in FIG. 4. As ions pass through filter 24between filter electrodes 20 and 22, the individual ions 19-19 may bedetected spatially, the ions having their trajectories 42 c-42 cdetermined according to their size, charge and cross section. Thusdetector segment 33′ will have a concentration of one species of ionwhile detector segment 33 will have a different ion speciesconcentration, increasing the spectrum resolution as each segment maydetect a particular ion species.

[0069] A PFAIMS spectrometer as set forth herein is able to detect anddiscriminate between a wide range of compounds, and can do so with highresolution and sensitivity. As shown in FIG. 5A, varying concentrationsof acetone that were clearly detected in one practice of the invention,with definitive data peaks 46 at −3.5 volts compensation. These weredetected even at low concentrations of 83 parts per billion. With thebias voltage set at −6.5 volts, FIG. 5B, varying concentrations ofdiethyl methyl amine were clearly detected in practice of the invention,generating data peaks 46; these were detected in concentrations as lowas 280 parts per billion.

[0070] Turning to FIG. 6 and FIG. 7, an embodiment of spectrometer 10includes spaced substrates 52 and 54, for example glass or ceramic, andelectrodes 20 and 22, which may be for example gold, titanium, orplatinum, mounted or formed on substrates 52 and 54, respectively.Substrates 52 and 54 are separated by spacers 56a-b which may be formedby etching or dicing silicon wafer. The thickness of spacers 56a, 56bdefines the distance between electrodes 20 and 22.

[0071] In one embodiment, a voltage is applied to silicon spacers 56a-b, ±(10-1000 volts dc), which transforms spacers 56 a and 56 b intoelectrodes to produce a confining electric field 58. Field 58 guides orconfines the ions' paths to the center of flow path 26 in order toobtain more complete sample collection. As will be understood by aperson skilled in the art, spacer electrodes 56 a-b must be set to theappropriate voltage so as to “push” the ions toward the center of flowpath 26. More ions traveling in the center of the path makes possiblethe result of more ions striking electrodes 33 and 35 for detection.However, this is not a necessary limitation of the invention.

[0072] Embodiments of the invention are compact with low parts count,where the substrates and spacers act to both contain the flow path andalso to for a structural housing of the invention. Thus the substratesand spacers serve multiple functions, both for guiding the ions and forcontaining the flow path.

[0073] In order to further assure accurate and reliable operation ofspectrometer 10, neutralized ions which accumulate on electrode plates20 and 22 are purged. In one embodiment this may be accomplished byheating flow path 26. For example, controller 30, FIG. 1, may includecurrent source 29, shown in phantom in FIG. 6, which provides, inresponse to microprocessor 36, a current I to electrode plates 20 and 22to heat the electrodes for removing accumulated neutrals. Optionally,current I may additionally or instead be applied to spacer electrodes 56a and 56 b, to heat flow path 26 to purge electrodes 20 and 22.

[0074] A packaged FAIMS spectrometer 10 may be reduced in size toperhaps one inch by one inch by one inch. Pump 14 is mounted onsubstrate 52 for drawing gas sample 12 into inlet 16. Clean dry air maybe introduced into flow path 26 by recirculation pump 14 a prior to orafter ionization of the gas sample. Electronic controller 30 may beetched into silicon control layer 60 which combines with substrates 52and 54 to form a housing for spectrometer 10. Substrates 52 and 54 andcontrol layer 60 may be bonded together, for example, using anodicbonding, to provide an extremely small FAIMS spectrometer. Micro pumps14 and 14 a provide a high volume throughput which further expedites theanalysis of gas sample 12. Pumps 14 and 14 a may be, for example,conventional miniature disk drive motors fitted with small centrifugalair compressor rotors or micromachined pumps, which produce flow ratesof 1 to 4 liters per minute.

[0075] In practice of ion detection, generally speaking, sample ionstend to be found in either monomer or cluster states. It has been foundthat the relationship between the amount of monomer and cluster ions fora given ion species is dependent of the concentration of sample and theparticular experimental conditions (e.g., moisture, temperature, flowrate, intensity of RF-electric field). Both the monomer and clusterstates provide useful information for chemical identification. It willbe useful to investigate the same sample separately in a condition whichpromotes clustering and in an environment that promotes the formation ofonly the monomer ions. A two channel PFAIMS of an embodiment such asshown in FIG. 8 can be used toward this end.

[0076] Dual chamber embodiment 10 x of the invention, FIG. 8, has twoenclosed flow paths 26′, 26″ coupled by passageway 63. The gas sample 12enters inlet 16 a and is ionized at ionization region 17 in the lowerflow path 26′, ionized by any ionization device, such as an internalplasma source 18 a. The ions are guided toward ion filter 24 a in upperflow path 26″ through passageway 63 by electrodes 56 ax and 56 bx, whichact as steering or deflecting electrodes, and may be defined byconfining electrodes 56 a, 56 b. As these ions 42 c pass between ionfilter electrodes 20 a and 22 a, undesirable ions will be neutralized byhitting the filter electrodes while selected ions will pass throughfilter 24 a to be detected by detector 32 a, according to the applied RFand compensation. By deflecting ions out of the gas flow, a preliminaryfiltration is effected, wherein the non-deflected ions and non-ionizedsample and associated carrier gas will be exhausted at outlet 16 x′. Theexhaust gas 43 from upper flow path 26″, at outlet 16 x″, may becleaned, filtered and pumped via pump part 14 a and returned at inlet 16b as clean filtered gas 66 back into the flow path 26″.

[0077] In one practice of the invention, clean dry air 66a may beintroduced into flow path 26 through clean air inlet 66 via pump 14.Drawing in clean dry air assists in reducing the FAIMS spectrometer'ssensitivity to humidity. Moreover, if the spectrometer is operatedalternately with and without clean dry air, and with a known gas sampleintroduced into the device, then the device can be used as a humiditysensor since the resulting spectrum will change with moistureconcentration from the standardized spectrum for the given known sample.

[0078] In operation of the embodiment of FIG. 8, independent control ofthe flow rates in flow paths 26′, 26″ may be made. This means that ahigher or lower flow rate in flow path 26′ of the sample can be used,depending on the particular front end environment system, while the flowrate of the ions through the ion filter in flow path 26″ can bemaintained constant, allowing, consistent, reproducible results.

[0079] In practice of this embodiment, the upper ion filter region inflow path 26″ can be kept free of neutrals. This is important whenmeasuring samples at high concentrations, such as those eluting from aGC column. Because the amount of ions the ionization source can provideis fixed, if there are too many sample molecules, some of the neutralsample molecules may cluster with the sample ions and create largemolecules which do not look at all like the individual sample molecules.By injecting the ions into the clean gas flow in flow path 26″, and dueto the effect of the high voltage high frequency field, the moleculeswill de-cluster, and the ions will produce the expected spectra.

[0080] Another advantage of the embodiment of FIG. 8 is that the dynamicrange of the PFAIMS detector can be extended when employing a front enddevice (such as a GC, LC or electrospray for example). In one practiceof the invention, by adjusting the ratios of the drift gas andGC-sample/carrier gas volume flow rates coming into ionization region17, the concentration of the compounds eluting from the GC can becontrolled/diluted in a known manner so that samples are delivered tothe PFAIMS ion filter 24 at concentrations which are optimized for thePFAIMS filter and detector to handle. In addition, steering electrodes56 ax, 56 bx can be pulsed or otherwise controlled to determine how manyions at a given time enter into flow path 26″.

[0081] In a further practice of the embodiment of FIG. 8, an additionalPFAIMS filter 24 b may be provided in lower flow path 26′ for detectionof ion species that have not been deflected into flow path 26″ and thusthat pass into filter 24 b. Filter 24 b includes electrodes 20 b, 22 b,shown in phantom, and possibly also detector 32 b having electrodes 33b, 35 b, in phantom.

[0082] In the embodiment of FIG. 8, different gas conditions may bepresented in each flow path. With a suitable control applied to the twosteering electrodes 56 ax, 56 bx, selection can be made as to whichregion the ions are sent. Because each chamber can have its own gas andbias condition, multiple sets of data can be generated for a singlesample simultaneously. This enables improved species discrimination in asimple structure, regardless of whether or not a front end device (suchas a GC) is used for sample introduction.

[0083] One prior art ion mobility spectrometer 200, FIG. 9, (See U.S.Pat. No. 5,420,424), includes analytical gap 202 defined by the spacebetween inner cylindrical filter electrode 204 and outer cylindricalfilter electrode 206 electrodes. A source gas having compounds to beanalyzed is drawn through inlet 210 via the action of pump 212; thesample is ionized by ionization source 214. A carrier gas CG isintroduced via pump 216 into analytical gap 202. Ions generated byionization source 214 travel through aperture 218 by the action ofelectrode 220 and into analytical gap 202 and travel toward detector224. Such a structure requires two pumps 212 and 216, and separate flowpaths 201 and 203 for the source gas and the carrier gas. Thus, priorart mobility spectrometer 200 cannot be made very small, and requiressufficient power to operate the pumps 212 and 216.

[0084] Embodiments of the present invention overcome limitations of theprior art by providing field-driven ion transport via an ion flowgenerator, where ions flow through an ion filter as carried by the iontransport field. The ion flow generator of the present inventionrelieves the gas flow requirements of the prior art. Various options arepossible, including providing a low volume flow, no gas flow, or reversegas flow, along the longitudinal axis of the flow path. The reverse flowcan be a supply of clean filtered air in the negative z direction tokeep the ion filter and detector regions free of neutrals and to helpremove solvent, reduce clustering, and minimize the effects of humidity.The ion flow generator is preferably based on electric potentials, butmay be practiced in magnetic embodiments, among others, and still remainwithin the spirit and scope of the present invention. Variousembodiments follow by way of illustration and not by way of limitation.

[0085] In one practice of the invention, shown in FIG. 10, fieldasymmetric ion mobility spectrometer 230 includes a flow path 231 insidehousing structure 234 (which may be formed by a round tube or a flathousing with walls defining an enclosure). A source gas carries sample Sinto the ionization region near the ionization source 236. This flow issupplied by pump 238, which may be a micromachined pump with a flow rateof much less than the typically required 1-4 liters per minute of theprior art (resulting in a power savings of between 1-5 watts over priorart spectrometers). Alternatively, this flow might be supplied by sampleeluting from a GC column or the like.

[0086] Ion filter 240 is disposed in flow path 231 downstream fromionization source 236. Ion filter 240 creates the asymmetric electricfield 242 (a compensated field 25), to filter ions generated byionization of sample S. Ion filter 240 may include a pair of spacedelectrodes 248 and 246 connected to an electric controller which appliesa compensated asymmetric periodic voltage to electrodes 246 and 248.

[0087] In spectrometer 230, ion flow generator 250 provides longitudinalelectric field transport for the ions. The strength of longitudinalelectric field 252 can be constant or varying in time or space; thefield propels ions through the filter asymmetric field 242, with ionspassing through the filter according to their characteristics and thefilter field compensation.

[0088] In the embodiment of FIG. 10, ion flow generator 250 includesdiscrete electrodes 260, 262, 264, and 266 supported by and insulatedfrom filter electrode 246 by insulating medium 268, and discreteelectrodes 261, 263, 265,. and 267 supported by and insulated fromfilter electrode 248 by insulating medium 269. In one practice of theinvention, electrodes 260, 261 are at 1,000 volts and electrodes 266,267 are at 10 volts and electrode pairs 262, 263 and 264, 265 are at 500and 100 volts, respectively, although these voltage levels vary or bevarying depending on the specific implementation of spectrometer 230.There may be more or fewer electrodes opposing each other forming ionflow generator 250. Electrode pairs (260, 261), (262, 263), (264, 265),and (266, 267) can also each be a ring electrode as well as discreteplanar electrodes. The strength of longitudinal electric field 252propels ions generated at ionization source 236 through asymmetricelectric field 242 and toward detector 270, thus eliminating or reducingthe flow rate and power requirements of pumps 212 and 216, FIG. 9 of theprior art.

[0089] Typically, detector 270 (which may have the configuration shownearlier of two electrodes 33, 35 on substrates 52, 54) is positionedclose to ion flow generator 250. Electrodes 260, 262, 264, 266, 261,263, 265, and 267 preferably occupy more or less the same longitudinalspace as ion filter 240 and its electrodes 246 and 248 relative to a gap232 in flow path 231.

[0090] In the embodiment of the invention shown in FIG. 11, ion filter240 includes spaced electrodes 276 and 277 for creating transversefilter field 242. The ion flow generator 250 includes spaced discreteelectrodes, such as electrode pairs 282-284 and 286-288, for generatinglongitudinal transport field 252. In one practice, electrodes 282 and284 are at 1000 volts and electrodes 286 and 288 are at 1000 volts.Insulating medium 290 and 291 insulates electrodes 282, 284, 286, and288 with respect to electrodes 276 and 277. Electrode pair 282-284 couldalso be coupled as a single ring electrode and electrode pair 286-288could be coupled also be a single ring electrode in a cylindricalembodiment of the invention.

[0091] It will be appreciated that the sample must be conveyed to theionization region and the ions must be conveyed into the filter. In thedesign of FIG. 11, the ions are propelled by a low volume flow along thedirection of the longitudinal electric field 252 to bring the ionsproximate to electrodes 282-284. No gas flow is required in the ionfilter and detector region due to longitudinal electric field 252. Alsoin this embodiment, a low flow volume of clean filtered air optionallycan be provided in a direction opposite the longitudinal electric fieldto keep the ion filter and detector region free of neutrals. A resistivedivider circuit or the like can be used to provide a potential gradient,so that for example, electrodes 282 and 284 are at 1000 volts whileelectrodes 286 and 288 are at 0 volts.

[0092] An alternative practice of the invention is shown in FIG. 12,having metal filter electrodes 276, 277 deposited on insulatingsubstrates 310, 311 and filter electrodes 276, 277 coated with a thininsulator 290, 291. Metal electrodes, e.g., 312, 314, 316, 318, areformed under a resistive layer 300, 302, and the longitudinal field isgenerated between these electrodes. In one practice, ion filter 240includes spaced resistive layers 300 and 302 insulated from electrodes276 and 277 on insulating substrates 310, 311 by insulating medium 290and 291, for example, a low temperature oxide material. Resistive layers300 and 302 may be a resistive ceramic material deposited on insulatinglayers 290 and 291, respectively. Terminal electrodes 312, 314, 316 and318 make contact with each resistive layer to enable a voltage dropacross each resistive layer that generates the longitudinal electricfield 252, for example, where electrodes 312 and 316 are at 1000 voltswhile electrodes 314 and 318 are at 0 volts. This embodiment can beextended to a cylindrical geometry by making electrodes 312 and 316 aring electrode, electrodes 314 and 318 a ring electrode, and resistivelayers 300 and 302 an open cylinder.

[0093] Continuing with the benefits of a dual flow path, such as earliershown in FIG. 8, in the embodiment of FIG. 13 spectrometer 320 includesstructure which also defines dual flow paths 321, 323. Ion filter 240and ion flow generator 250 are defined by sets of electrodes in thisembodiment. Gap 304 is defined in flow path 323 at filter 240. Opening306 joins the flow paths. Source gas carrying sample S to be analyzed isdrawn into flow path 302 by pump 310 and ionized by ionization source308. The ions are deflected through opening 306 and into gap 304assisted by deflecting electrodes 312 and 313. Ion flow generator 250propels the ions through the asymmetric ion field at filter 240.Optionally pump 312 can be used to supply a low flow rate of air,possibly dehumidified, into, or recirculating through, gap 304, but nocarrier gas flow is required in flow path 302. Ion species passed by thefilter are carried by the ion transport 252 to detector 270.

[0094] In another embodiment of the invention, shown in FIG. 14,spectrometer 325 includes a desiccant 322 chambered in housing 326 andsmall pump 324, which is the only pump required to draw source gas intohousing 326 through a small orifice 327. Ionization source 328 producesions which travel through filter 240 aided by the longitudinal electricfield created by ion flow generator 250. The desiccant serves to furthercondition the sample gas before filtering for improved performance.

[0095] In still another embodiment shown in FIG. 15, spectrometer 333includes ion filter 240 with a plurality of RF electrodes 340, 342, 344and 346 connected to an electric controller 30 which applies theasymmetric periodic voltage to create the filtering field. DCcompensation may also be applied to these electrodes. The ion flowgenerator 250 includes a second plurality of discrete electrodes 348,350, 352 and 354 dispersed among but insulated from the discrete RFelectrodes of the ion filter and connected to controller 30, whichestablishes a gradient between the electrodes to generate an ionpropelling transport field 252 along the flow path toward the detector270. The electrodes may be coated with an insulating material 358, aswell as being isolated from each other by adequate insulation.

[0096] In the embodiment of FIG. 15, all the RF electrodes may beindependently driven or tied together while the longitudinal fieldproducing electrodes have a potential gradient dropped across them. Inone embodiment, the voltages applied to the electrodes can be alternatedso that first a voltage is applied to generate the transverse RFelectric field 242 and then a voltage is applied to other or sameelectrodes to generate the longitudinal ion transport field 252.

[0097] In still another embodiment, spectrometer embodiment 359 shown inFIG. 16 includes RF electrodes 360, 362, which provide the asymmetricion filtering electric field 252 are disposed on the outside walls ofinsulative substrates 52, 54. Resistive layers 370 and 372 may be aresistive ceramic material deposited on the inside walls of insulatingsubstrates 52 and 54, respectively. Terminal electrodes 374-375, and377-378 make contact with each resistive layer is shown to enable avoltage drop across each resistive layer to generate the ion propellinglongitudinal electric field 252. Thus, electrodes 374 and 377 may eachbe at −100 volts while electrodes 375 and 378 are at −1000 volts, forexample.

[0098] In the embodiment of FIG. 17, spectrometer 379 has discreteelectrodes 380-386 on substrate 52 and 387-394 on substrate 54 whichcooperate to produce an electrical field or fields. The net effectprovides both transverse and longitudinal field components to bothfilter and propel the ions. A traveling wave voltage of the form

Vcos (wt−kz)

[0099] where k=2π/λ is the wave number has an associated electric fieldwith both transverse and longitudinal components 242+252. For a planarsystem, each succeeding set of opposing electrodes is excited by avoltage source at a fixed phase difference from the voltage sourceapplied to the adjacent set of opposing electrodes.

[0100] Thus, electrodes 380 and 387 are excited with a voltage ofvcos(wt) while electrodes 381 and 388 are excited with a voltage of vcos(wt+120) and so on as shown in FIG. 17. Traveling wave voltages requiremultiphase voltage excitations, the simplest being a two phaseexcitation. So, a two conductor ribbon could also be wound around a ductdefining the gap with one conductor excited at vcos (wt) and the otherconductor excited at vsin (wt). Three phase excitations could beincorporated if the conductor ribbon or tape had three conductors.

[0101] In an alternative of the embodiment of FIG. 17, the discreteelectrodes 380-386 and 387-394 are still driven to produce bothtransverse and longitudinal fields to both filter and propel the ions.The PFAIMS RF signal is applied to the electrodes to generate thetransverse RF field, which may involve one electrode on each substrateor multiple electrodes. Compensation is also generated, either byvarying the duty cycle or the like of the RF, or by applying a DC biasto the electrodes, which may involve one electrode on each substrate ormultiple electrodes. Finally, the ion flow generator includes aselection of these electrodes biased to different voltage levels (e.g.,1000 vdc on electrodes 380 and 387 and 100 vdc on electrodes 386 and393) to generate a gradient along the flow path. Compensation voltageapplied to the RF filter field opens the filter to passage of a desiredion species if present in the sample as propelled by the flow generator.If the compensation voltage is scanned, then a complete spectrum of thecompounds in a sample can be gathered.

[0102] In a further embodiment of the invention, ion filtering isachieved without the need for compensation of the filter field. As shownin FIG. 18, in one illustrative embodiment, spectrometer 410 has asingle RF (high frequency, high voltage) filter electrode 412 onsubstrate 52. A segmented filter-detector electrode set 414 on substrate54 has a plurality of electrodes 414 a-414 n. Electrode 412 faces set414 over flow path 26. Strips 414 a-414 n are maintained at virtualground, while the asymmetric field signal is applied to the filterelectrode 412.

[0103] It will be further appreciated that, referring to FIG. 2, ions 19flow in the alternating asymmetric electric field 25 and travel inoscillating paths that are vectored toward collision with a filterelectrode, and collision will occur in absence of adequate compensation.In the embodiment of FIG. 18, the absence of compensation favorablyenables driving of the ions to various electrodes of the segmentedelectrode set 414. Thus all of the ions are allowed to reach and contactone of the electrodes 414 a-414 n. These ions thus deposit their chargesupon such contact, which is monitored such as with current meters 421,421. (It will be further appreciated that this arrangement isillustrative and not limiting. For example, the filter electrode may besegmented, similar to the filter-detector electrode set, where ions alsowill be detected thereon.)

[0104] In an illustrative embodiment, upstream biasing effects whichions flow to the filter. For example, a sample S flows (“IN”) into anionization region 415 subject to ionization source 416. Electrodes 417,418, 419 are biased to deflect and effect flow of the resulting ions.Positive bias on electrode 419 repels positive ions toward the filterand electrodes 417, 418 being negatively biased attract the positiveions into the central flow of filter 420, while negative ions areneutralized on electrode 419 and which are then swept out (“OUT”) of theregion. Negative bias on electrode 419 repels negative ions toward thefilter and electrodes 417, 418 being positively biased attract thenegative ions into the central flow path 26 of filter 420, whilepositive ions are neutralized on electrode 419.

[0105] The path taken by a particular ion in the filter is mostly afunction of ion size, cross-section and charge, which will determinewhich of the electrodes 414 a-414 n a particular species will driveinto. This species identification also reflects the polarity of the ionsand the high/low field mobility differences (“alpha”) of those ions.Thus a particular ion species can be identified based on its trajectory(i.e., which electrode is hit) and knowledge of the signals applied, thefields generated, and the transport characteristics (such as whether gasor electric field).

[0106] In practice of the filter function, where the upstream biasingadmits positive ions 19+ into the filter, those positive ions with analpha less than zero will have a mobility decrease with an increase ofthe positively offset applied RF field (waveform 25 a). This will effectthe trajectory of these ions toward downstream detector electrode 414 n.However, a positive ion 19+ with an alpha greater than zero will have amobility increase with an increase of the negatively offset applied RFfield (waveform 25 b), which in turn will shorten the ion trajectorytoward the nearer detector electrodes.

[0107] Similarly, where the filter is biased to admit negative ions, anegative ion 19− with an alpha less than zero will have a mobilityincrease with an increase of the positively offset applied RF fieldwaveform 25 a; this will tend to effect the ion trajectory towarddownstream detector electrode 414 n. However, a negative ion 19− with analpha greater than zero will have a mobility increase with an increaseof the negatively offset applied RF field waveform 25 b, which in turnwill tend to shorten the ion trajectory toward the nearer detectorelectrodes. Thus, ions can be both filtered and detected in spectrometer410 without the need for compensation.

[0108] Various embodiments of the present invention are able to identifycompounds in a chemical sample down to trace amounts. In FIG. 19,identification of individual constituents of a mixture is demonstratedby the distinct and separate Benzene peaks 422 and acetone peaks 424obtained in practice of the invention. Three plots are superimposed inFIG. 19. The first plot is for benzene and acetone (1-3) ppm; the secondplot is for benzene and acetone (trace). The bottom plot shows benzenealone. It therefore can be observed that the acetone peak can be easilydistinguished from the benzene peak in practice of the presentinvention. This capability enables separation and identification of awide array of compounds in chemical samples in a compact andcost-effective method and apparatus of the invention.

[0109] Multiple use of electrodes is not limited to the examples setforth above. Embodiments of the present invention lend themselves to theuse of an electrospray ionization source nozzle because certain of theelectrodes can function both as the source for the PFAIMS andlongitudinal electrical field which transports the ions toward thedetector electrodes, but also as the electrospray electrodes whichcreate a fine spray sample for ionization. Thus, in accordance with thepresent invention, pumps 216 and 212, FIG. 9 of the prior art are eithereliminated or at least reduced in size and have lower flow rate andpower requirements.

[0110] In practice of the invention, by the incorporation of an ion flowgenerator which creates a longitudinal electric field in the directionof the intended ion travel, the ions are propelled through thetransversely directed compensated asymmetric electric field and onwardfor detection. The apparatus may include a detector or may deliver ionsto a detector.

[0111] In practice of the invention, pump and gas flow requirements aresimplified. By eliminating the high flow rate of pumps used in prior artspectrometers, a significant reduction in power consumption, size, andcost can be realized leading to a miniaturized spectrometer on a chip inpractice of embodiments of the invention.

[0112] Another benefit in practice of alternative embodiments of theinvention is that a flow of clean filtered air can be applied in adirection opposite the direction of the motion of the ions. In this way,any neutrals in the sample gas which were not ionized are deflected awayand do not enter the ion analysis region. The result is the reduction orelimination of ion clustering, and reduction of the impact of humidityon sensor performance. Because the flow rates are low, it is possible toincorporate integrated micromachined components. Molecular sieves can belocated close to the filter in order to absorb any neutral molecules inthe analysis region to reduce or prevent clustering.

[0113] Embodiments of the present invention employ a field asymmetricion mobility filtering technique that uses compensated high frequencyhigh voltage waveforms and longitudinal e-field propulsion. The RFfields are applied perpendicular to ion transport, with a planarconfiguration, but coaxial, concentric, cylindrical and radialembodiments are also within the scope of the invention.

[0114] The spectrometer can be made extremely small, if required, andused in chemical and military applications, as a filter for a massspectrometer, as a detector for a gas chromatograph, as a front end to atime of flight ion mobility spectrometer for increased resolution or asa filter for a flexural plate wave device.

[0115] The present invention provides improved chemical analysis. Thepresent invention overcomes cost, size or performance limitations of MS,TOF-IMS, FAIMS, and other prior art devices, in novel method andapparatus for chemical species discrimination based on ion mobility in acompact, fieldable packaging. These devices have the further ability torender simultaneous detection of a broad range of species, and have thecapability of simultaneous detection of both positive and negative ionsin a gas sample. Still further surprising is that this can be achievedin a cost-effective, compact, volume-manufacturable package that canoperate in the field with low power requirements and yet it is able togenerate definitive data that can fully identify various detectedspecies.

[0116] The present invention may be implemented using conventional oradvanced manufacturing techniques, such as MEMS or micromachining. Thesetechniques may include, for example, etching of smooth channels,chambers, dams, and intersections, and ports, forming and building uponsubstrates, etching and bonding, including anodic bonding and fusion,thin film processing and metallization applications, quartz machining,reactive ion etching, high temperature fusion bonding, photolithography,wet etching and the like.

[0117] Examples of applications for the present invention includechemical sensors and explosives sensors, and the like. Variousmodifications of the specific embodiments set forth above are alsowithin the spirit and scope of the invention. For example, it will befurther appreciated that embodiments of the invention may be practicedwith coaxial, concentric, ring, cylindrical, radial or other features.For example, the electrodes of FIG. 17 may be ring electrodes; as well,structural variations may appear in combination, such as where theelectrodes of FIG. 11 are ring electrodes and the remaining layers andelectrodes are coaxial and cylindrical, for example.

[0118] The examples disclosed herein are shown by way of illustrationand not by way of limitation. Although specific features of theinvention are shown in some drawings and not in others, this is forconvenience only as various features may be combined with any or all ofthe other features in accordance with the invention.

1. An asymmetric field ion mobility apparatus for identification of ionspecies, the apparatus comprising: an ion filter disposed in a flowpath, said flow path having a longitudinal axis for the flow of ions,said filter supplying an asymmetric filter field transverse to saidlongitudinal axis, said filter field being compensated; an ion flowgenerator for longitudinally propelling ions along said flow path insaid compensated asymmetric filter field; and the ion filter passing aspecies of said propelled ions, said species having a set ofcharacteristics correlated with said compensation, said correlationfacilitating identification of said species. 2-118. (Canceled)